Leaves and flowers and it will only my medicinal if it's a native version of Passiflora incarnata. But to harvest:

Harvest the long vines when they're begining to flower and haven't yet set much fruit. Let the vines sit in the shade for an hour to give the insects time to crawl off and inspect the vines for caterpillars. Hang to dry or prepare immediately as medicine

*The Healing Garden Juliet Blankespoor (pg. 346)